Silicon-germanium (SiGe) heterobipolar technology has generated worldwide interest for digital, analog and RF applications because it combines the transistor performance of III-V technologies with the manufacturability, high yield and low cost associated with conventional silicon integrated circuit (IC) fabrication. At present, SiGe technology development is mainly focused on npn SiGe heterobipolar transistors. However, for high-speed analog and mixed-signal circuit applications a complementary (npn and pnp) bipolar technology offers significant performance advantages over an npn-only technology. Push-pull circuits, for instance, ideally require a high-speed vertical pnp transistor with comparable performance to the npn transistor.
Because of the reduced mobility of holes on silicon, shallower base profiles are required for pnp heterobipolar transistors to achieve an AC performance comparable with that of an npn heterobipolar transistor. Due to the reduced diffusivity and high activation in silicon, arsenic should be the ideal dopant for ultra-shallow pnp SiGe epitaxial base layers. Typically, in epitaxial SiGe base layers the Ge profile overlaps with the dopant profile so that in order to have reproducible process results the interaction between the dopant and Ge has to be minimized. Unfortunately, in case of arsenic as a dopant it has turned out that there is a strong correlation of the arsenic concentration and the SiGe film growth-rate with arsenic flow for nearly all germanium concentrations which would be within the interest of typical silicon-germanium heterobipolar transistors, i.e. germanium concentrations between 1% and 15%.